2.1. LIPOSOMES
Liposomes are completely closed bilayer membranes containing an encapsulated aqueous phase. Liposomes may be any variety of multilamellar vesicles (onion-like structures characterized by concentric membrane bilayers each separated by an aqueous layer) or unilamellar vesicles (possessing a single membrane bilayer).
Two parameters of liposome preparations are functions of vesicle size and lipid concentration: (1) Captured volume, defined as the volume enclosed by a given amount of lipid, is expressed as units of liters entrapped per mole of total lipid (1 mol.sup.-1). The captured volume depends upon the radius of the liposomes which in turn is affected by the lipid composition of the vesicles and the ionic composition of the medium. (2) Encapsulation efficiency, defined as the fraction of the aqueous compartment sequestered by the bilayers, is expressed as a percentage. The encapsulation efficiency is directly proportional to the lipid concentration; when more lipid is present, more solute can be sequestered within liposomes. (See Deamer and Uster, 1983, Liposome Preparation: Methods and Mechanisms, in Liposomes, ed. M. Ostro, Marcel Dekker, Inc., N.Y., pp. 27-51.)
The original method for liposome preparation (Bangham et al., 1965, J. Mol. Biol. 13: 238-252) involved suspending phospholipids in an organic solvent which was then evaporated to dryness leaving a waxy deposit of phospholipid on the reaction vessel. Then an appropriate amount of aqueous phase was added, the mixture was allowed to "swell," and the resulting liposomes which consisted of multilamellar vesicles (hereinafter referred to as MLVs) were dispersed by mechanical means. The structure of the resulting membrane bilayer is such that the hydrophobic (non-polar) "tails" of the lipid orient toward the center of the bilayer while the hydrophilic (polar) "heads" orient towards the aqueous phase. This technique provided the basis for the development of the small sonicated unilamellar vesicles (hereinafter referred to as SUVs) described by Papahadjopoulos and Miller (1967, Biochim. Biophys. Acta. 135: 624-638). Both MLVs and SUVs, however, have limitations as model systems.
In attempts to increase captured volume or encapsulation efficiency a number of methods for the preparation of liposomes comprising physpholipid bilayers have been developed; however, all methods require the use of organic solvents. Some of these methods are briefly described below.
An effort to increase the encapsulation efficiency involved first forming liposome precursors or micelles, i.e., vesicles containing an aqueous phase surrounded by a monolayer of lipid molecules oriented so that the polar head groups are directed towards the aqueous phase. Liposome precursors are formed by adding the aqueous solution to be encapsulated to a solution of polar lipid in an organic solvent and sonicating. The liposome precursors are then emulsified in a second aqueous phase in the presence of excess lipid and evaporated. The resultant liposomes, consisting of an aqueous phase encapsulated by a lipid bilayer are dispersed in aqueous phase (see U.S. Pat. No. 4,224,179 issued Sep. 23, 1980 to M. Schneider).
In another attempt to maximize the encapsulation efficiency, Papahadjopoulos (U.S. Pat. No. 4,235,871 issued Nov. 25, 1980) describes a "reverse-phase evaporation process" for making oligolamellar lipid vesicles also known as reverse-phase evaporation vesicles (hereinafter referred to as REVs). According to this procedure, the aqueous material to be encapsulated is added to a mixture of polar lipid in an organic solvent. Then a homogeneous water-in-oil type of emulsion is formed and the organic solvent is evaporated until a gel is formed. The gel is then converted to a suspension by dispersing the gel-like mixture in an aqueous media. The REVs produced consist mostly of unilamellar vesicles (large unilamellar vesicles, or LUVs) and some oligolamellar vesicles which are characterized by only a few concentric bilayers with a large internal aqueous space.
Much has been written regarding the possibilities of using liposomes for drug delivery systems. See, for example, the disclosures in U.S. Pat. No. 3,993,754 issued on Nov. 23, 1976, to Yeuh-Erh Rahman and Elizabeth A. Cerny, and U.S. Pat. No. 4,145,410 issued on Mar. 20, 1979, to Barry D. Sears. In a liposome drug delivery system the medicament is entrapped during liposome formation and then administered to the patient to be treated. The medicament may be soluble in water or in a non-polar solvent. Typical of such disclosures are U.S. Pat. No. 4,235,871 issued Nov. 25, 1980, to Papahadjopoulos and Szoka and U.S. Pat. No. 4,224,179, issued Sep. 23, 1980 to M. Schneider. When preparing liposomes for use in vivo it would be advantageous (1) to eliminate the necessity of using organic solvents during the preparation of liposomes; and (2) to maximize the encapsulation efficiency and captured volume so that a greater volume and concentration of the entrapped material can be delivered per dose.